Fix sequence publication race in allow-write external SST ingestion

[LykxSassinator]

Summary

IngestExternalFiles(..., allow_write=true) allowed foreground writes to run while external SST ingestion assigned and published global sequence numbers. A concurrent multi-key WriteBatch could complete after ingest read LastSequence() but before ingest published its consumed sequence number, causing the visible sequence to move backwards. New RocksDB snapshots could then miss committed data.

This change restores serialization only for the final ingestion phase:

• Keep writes enabled during prepare, the initial overlap check, and any initial flush.
• Enter the write-thread gates before Run() and wait for pending writes.
• Recheck overlap under the gates and perform a fallback flush if needed.
• Hold the gates through Run(), LogAndApply, sequence publication, and SuperVersion installation.

This preserves most of the latency benefit of allow_write while restoring the required global sequence linearization.

Tests

• Added a deterministic sync-point regression test covering the stale sequence publication ordering.
• Verified both default and two_write_queues paths, with and without atomic_flush.
• Added explicit post-operation snapshot assertions for both WriteBatch keys and the ingested key.
• make -j4 external_sst_file_test
• Targeted tests: 8/8 passed
• Full external_sst_file_test: 99/99 passed
• git diff --check
• clang-format --dry-run --Werror

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